The present invention relates to a carburetor for IC engines. More specifically, the invention relates to a carburetor comprising an intake duct opening at one end into the atmosphere and connected at the other end with an intake pipe of a manifold of an IC engine. A throttle valve is located in the intake duct so as to essentially completely shut off same in an idling position, and an idling duct bypasses the throttle valve. The idling duct is formed to supply combustion air for the formation of a desired.fuel-air mixture, the fuel being caused to flow by vacuum of the combustion air at a fuel outlet from a fuel duct. The idling duct is formed with a bore constriction upstream from the outlet orifice of the idling duct in the intake duct for the production of a supersonic flow.
The invention also relates to an idling insert for such a carburetor which has a housing with a housing body for supporting a connector for fuel and a connector for combustion air. The insert comprises an inner fuel tube connected flow-wise with the fuel connector and an external jet tube connected flow-wise with the connector for combustion air. The jet tube extends, concentrically around the fuel tube with the formation of a support section for support in the carburetor housing, in a direction away from the housing body.
A carburetor of this type has been proposed in German Patent Specification No. 2,452,342. In this known carburetor, an idling duct was located within the material of the carburetor housing and primary air was supplied to the fuel in a vertically placed fuel duct through a branch duct arranged at an acute angle for the formation of an emulsion. At the lower end of the fuel duct the emulsion first passed into a plenum for transfer holes, which opened into the intake duct at the position at which the edge of the throttle valve made contact in its closed position. At the side of the plenum opposite the inlet, the fuel duct ran into a further plenum of the idling system, from which the fuel passed through a throttle duct, able to be set by means of a set screw and a projection on the throttle to change its cross section, to a mixing chamber for the addition and mixing of combustion air. On the side of the mixing chamber opposite to the throttle hole the fuel-air mixture passes into a small tube extending under the throttle valve a long distance into the intake duct and on its side adjacent to the mixing chamber it has a stepped choke structure which defines a bore constriction for producing a sonic velocity in the flow in the idling duct. The combustion air is supplied to the mixing chamber from an intake port in the wall of the intake duct over the throttle valve via a choke, which is responsible for the degree of vacuum requisite for causing flow of the fuel from the fuel duct.
The degree of vacuum produced in this way in the mixing chamber is considerable, since it has to cause the flow of the fuel out of the adjacent choke port at a velocity which, even initially, was relatively high, and such fuel has to be supplied on the other side of the mixing chamber to the narrow inlet of the choke structure without the mixing chamber being fouled by carbonizing condensate on its wall faces. This has to be achieved despite considerable pressure losses in the plenum chamber for the transfer holes and in the plenum chamber for the idling system with the throttle projection. Consequently, it is necessary for the degree of vacuum of the combustion air in the mixing chamber to be quite high.
On the other hand, however, the pressure in the mixing chamber always has to be just twice the pressure in the induction duct if a sonic velocity in the bore constriction is desired. Therefore, if one assumes a pressure in the mixing chamber of 0.75 bar, at which the flow of the emulsion caused is just adequate, the necessary pressure in the intake duct will be only approximately 0.4 bar at the most in order to attain a sonic velocity at the constriction with the ensuing relatively fine atomization desired. Although it may be possible under ideal conditions to construct this carburetor to attain this result during idling, this can be done only under the proviso that the degree of closing of the throttle valve is high and is not impaired by inaccuracies of manufacture or other factors, and under the further proviso that the rated idling speed of the engine is in fact attained; the idling speed may fall to a marked extent on switching on accessories requiring power such as an air conditioning system, a servosystem acting against an abutment or similar equipment and when this occurs the ideal conditions would no longer be fulfilled. A particular reason for such ideal conditions not being complied with is that, when changing over to a partial load phase of operation, the throttle valve is opened only a fraction so that the vacuum in the induction duct falls to some extent and the critical pressure ratio requisite for attaining sonic flow is no longer able to be reached.
The necessary consequence of this is that, even in the lower partial load range, desired fine atomization is no longer possible and, even during idling pure and simple, the set condition may very easily be lost so that even the automatic switching on of the air conditioning system may cause the engine to stop. However, in addition, even under ideal conditions which are not able to be permanently adhered to in practice at any rate for any length of time, there is only an incomplete amount of carburetion (in the sense of reducing the fuel droplet diameter down to an almost molecular order of size), because the combustion air exists at a low pressure and moves with a low velocity, and is combined in the mixing chamber with the emulsion (which also enters the mixing chamber as well with a relatively low velocity), such that, at the point of mixing, there will be no substantial effect to decrease the droplet diameter. Accordingly, the fuel-air mixture passes with a relatively large droplet diameter into the flow which is intended to be sonic, and it is only later that a reduction in the droplet diameter by the action of pressure waves is possible. Even if sonic flow is attained in the bore constriction, there will only be a limited degree of subsequent breaking down of the droplets in the mixture and if the sonic flow velocity is not reached, there will be a more or less complete absence of such breaking down of the droplet size.
From the original papers of German Pat. No. 2053991 issued to present inventor, it is also known to have an idling system using the transission between subsonic and supersonic flow to produce a vacuum for stimulating flow of fuel and air, and for intimate mixing and distribution thereof.